In modem aircraft fuel gauging systems, the primary sensing device is a capacitor. Capacitive fuel gauging systems are widely used for indicating the volume as well as the mass of fuel contained within fuel tanks. Such capacitance sensors have been widely accepted for many years because of their ruggedness and reliability.
A capacitive sensor, also referred to as a probe, is vertically arranged so that liquid fuel partially fills an inter-electrode space of the capacitor. The probe comprises two plates the electrical capacitance of which varies according to the relative permittivity of the fuel and the percentage of the inter-electrode volume filled with fuel. The probe is generally constructed of a pair of coaxial conductive cylinders which form the capacitor, although the technique is not limited to a cylindrical configuration. The probe is connected via a harness to a remote electronics unit. The remote electronics unit, also called a signal conditioner, performs a relative capacitance measurement to thereby produce a proportional voltage indicative of fuel level.
The process of measuring the fuel level with a capacitive probe becomes corrupted when stray capacitance from the wiring harness, which can vary temporally and among aircraft, adds to the desired capacitance in an intermittent and unpredictable way. Referring to FIG. 1, a diagram of a capacitive probe 10 connected to signal conditioning circuitry 12 is shown. A signal generator 13 provides a current on a line a to the variable capacitor 17. As described herein above, the capacitance of the capacitor 17 varies with respect to the level of fuel between the capacitor plates. A return current is provided via a diode 20 and a line b to a trans-resistance amplifier 22, which converts the return current into a voltage. The output of the amplifier 22 is provided to a detector 25, e.g., a peak detector, which provides an indication of the fuel level. A second diode 30 is provided in the probe between the capacitor and ground, via a line c, to provide DC restoration. The peak detector is responsive to the output of the amplifier 22 to provide an output voltage proportional to the composite current return from the probe. Thereafter, the output of the detector is provided to further signal conditioning circuitry to provide smoothing, empty tank offset voltage and full tank voltage adjust as is known in the art. The lines a, b and c represent the wire harness or cabling used to connect the probe 10 to the signal conditioning circuitry 12. C.sub.ab represents the stray capacitance between lines a and b, and i.sub.ab represents the current flowing through the stray capacitance C.sub.ab. Similarly, C.sub.ac represents the stray capacitance between lines a and c, and i.sub.ac represents the current flowing through the stray capacitance C.sub.ac.
Referring now to FIG. 2, the above probe and signal conditioning circuitry were tested with cables of different lengths to illustrate the effects of cable length and routing on the fuel reading. Cable lengths of 2 ft, 8 ft, and 16 ft (.61 m, 2.44 m, 4.88 m) were used. As can be seen from the test results, variations in cable length had a significant impact on the probe output due to stray capacitance effects.
Because of the uncertainty in measuring fuel with a variable capacitance probe, aircraft are required to maintain a larger fuel reserve than would be required if the fuel level was known with certainty. This uncertainty in fuel level has a significant impact on the range of an aircraft.